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Efforts have been made to utilize biologic parameters for determining optimal cardiac pacing rates. In this study of 67 patients, a significant relationship between heart rate and respiratory rate was observed during dynamic exercise. A system using a radiofrequency activator to modify pacing rate is described. Eleven patients have received VVI pacemakers with a similar implanted radio receiver coil. In two patients the fully automatic system has been successfully implanted. The experience with respiratory rate as a determinant of pacing rate is encouraging.

A pacemaker that adapts heart rate in response to the patient's metabolic requirements has been developed. The pacemaker uses breathing frequency and tidal volume as the indicators of physiologic demand. Maximal physical work capacity, anaerobic threshold, oxygen uptake (16 patients) and hemodynamic variables (9 patients) were assessed with fixed rate (VVI), atrial synchronous (VDT/I) and respiration-dependent ventricular (VVI-RD) pacing. All subjects attained their anaerobic threshold in stress tests with VVI pacing. The maximal physical capacity (p less than 0.001), work time to attain the anaerobic threshold (p less than 0.01) and oxygen uptake (p less than 0.001) were significantly greater with VVI-RD than with VVI pacing. The transition from the supine to the standing position was characterized by a significant increase of cardiac index at rest with both VDT/I and VVI-RD pacing as compared with VVI pacing. Progressive increments in the cardiac index and average left ventricular stroke work index were significantly different at submaximal and maximal exercise when VVI and VVI-RD were compared. At maximal exercise, mean cardiac output was also significantly different: 10.21 +/- 2.5 (SD) liters/min with VVI, 11.2 +/- 0.8 liters/min with VDT/I (p less than 0.05) and 12.65 +/- 3.1 liters/min with VVI-RD (p less than 0.05) pacing. Maximal oxygen extraction values were greater with VVI and VVI-RD pacing than with VDT/I pacing. Pulmonary artery end-diastolic pressures at maximal exercise were within the normal range with the three different modes of pacing. In conclusion, there is a significant (25%) improvement in exercise performance with VVI-RD pacing as compared with VVI pacing.(ABSTRACT TRUNCATED AT 250 WORDS)

A study was carried out to determine whether variations in the respiration rate during physical exercise could be used as a physiological variable in controlling the rate of an implanted pacemaker. The relation between respiration rate and heart rate was significantly correlated in 73 patients (19 with normal lung function, four with restrictive pulmonary disease, and 50 with obstructive airways disease) during repeated calibrated ergometric tests; no significant differences were found between the subgroups. An external computerised programmable system with algorithm control activated by a radio frequency system was used to vary the cardiac stimulation rate in relation to respiration rate in 11 patients implanted with ventricular inhibited pacemakers. In addition, a prototype programmable pacemaker dependent on respiration rate was implanted in two patients. Maximum values of oxygen uptake, minute ventilation, and work time were increased during the exercise stress tests when the variable cardiac pacing rate was used. Thus respiration rate appears to be a valid and stable physiological variable for controlling the cardiac stimulation rate in order to improve cardiac output in patients dependent on pacemakers.

The rate responsive pacemaker using respiratory rate as a sensor was implanted in 22 patients, 19 patients for ventricular pacing and three for atrial pacing. The level of exercise achieved with this system was consistently higher than with a fixed ventricular pacing rate. In addition, no special chemical or mechanical sensors are required; the sensor itself is simple, long-lasting, and energy efficient. Thus, this system adapts easily to the individual patient.